This invention relates to a process for mixing isocyanate and a polyol that reacts to form polyurethane used for the production of shoe soles, the isocyanate and the polyol being fed into the bore of an elongated cylinder having the discharge opening at one end, and the bore having a conical surface tapering toward that end. The isocyanate and the polyol are mixed into a mixture reacting into polyurethane by rotating an externally threaded worm in the bore. The worm has a shape complementary to the shape of the bore. The polyurethane is conveyed through the discharge opening into a shoe sole mold cavity by axially shifting the worm in one direction toward the discharge end after the mixing step.
German patent No. 2,062,008 discloses a mixing device for the mixing of isocyanate and a polyol into a mixture reacting into polyurethane, the device including a mixing worm that has a conical taper at a section adjacent the discharge end of a cylinder in which the worm operates. The bore of the cylinder has a shape complementary to the worm, and the worm is capable of axially shifting toward the discharge end of the cylinder for injecting the polyurethane into a shoe sole mold cavity.
This type of mixing device is used, for example, on shoe soling machines in which soles are molded on to shoe uppers, the soles comprising an outer sole of wear resistant polyurethane and an elastic inner sole of polyurethane. The mold machines are mounted on a circular turntable to define a plurality of spaced apart molding stations for the molding of the outer and inner soles on to the shoe uppers. The molding machine is located at each stations on the turntable, the stations being stopped during incremental movement and locked when stopped relative to two mixing and injection devices. These devices are located adjacent the periphery of the turntable and are movable in a radial direction toward the turntable. The molding machine at each station has two lateral molding elements which, in a mold closed position, define an opening and sprue channels. A vertically movable last turning body is mounted on the apparatus above the lateral mold elements, the body having a counter stamp and a shoe last for supporting the shoe uppers. A vertically adjustable bottom stamp is located below the lateral mold elements. The bottom stamp, the lateral mold elements in their mold closing position and the counter stamp together define a mold cavity into which the mixture reacting into the outer sole is introduced. The bottom stamp which contains the molded outer sole, the lateral mold elements in a mold closed position, and the shoe last which supports the shoe upper define a mold cavity for molding the inner sole as the mixture reacting into the inner sole is introduced into the mold cavity, which, in the course of the reaction bonds to the outer sole and to the shoe upper.
During the molding of the outer sole, the closed lateral mold elements, the bottom stamp and the counter stamp define a mold cavity the volume of which is somewhat larger than the volume of the outer sole of the reacted mixture, whereby the two lateral mold elements in their closed position surround a pouring bore through which the mixture for the outer sole is fed into the defined mold cavity. Then, the mold cavity is reduced to the volume of the finished outer sole, and the bottom stamp simultaneously closes off the feed bore. After sufficient solidification of the material for the outer sole, the sides of the mold are opened, the last turning body is pivoted and moved such that the last supported shoe upper is disposed at the reclosed lateral mold elements so that the lasted shaft, the closed lateral mold elements and the outer sole and the bottom stamp form a mold cavity for molding the intermediate sole, whereby the lateral mold elements surround a second feed bore through which the mixture reacting and forming the inner sole is fed in.
The aforedescribed molding operation is disclosed in U.S. Pat. No. 4,801,256, the entire disclosure of which is specifically incorporated herein by reference.
During mixing of the isocyanate and the polyol chemical reactions start so that a portion of the mixture precipitates on to the inner wall of the cylinder in which the worm operates and forms a film of polyurethane which, over time, becomes thicker. This results in reduction of the gap between the cylinder wall and the worm such that the gap required for mixing becomes closed. For this reason it is necessary to at least partially reduce the film to maintain the gap at a predetermined minimum gap width. When the film of reacted polyurethane forming on the cylinder bore wall has reduced the spacing between the cylinder bore and the worm to practically zero, the polyurethane film is removed and expelled outwardly of the bore by an axial shift of the worm carried out during the interval while the device is out of contact with the feed bore of the molds. The feed of isocyanate and polyol takes place through corresponding valves in measured amounts during the individual mixing intervals.
Although such a mixing and injection device disclosed in German patent No. 2,062,008 is capable of maintaining the minimum gap width during operation, problems arise in that the non-reacted mixture tends to dribble and drip from the discharge end of the cylinder during the material conveying operation. This is particularly disadvantageous when molds are used which are filled on the mold machine by a pouring process of the mixing device with given amounts of the component mixture.